It says buyers are usually savvy enough to question dodgy MPG figures.

Even so, it decided to perform a mystery shop at 100 UK car retailers in a bid to get an accurate idea of what salesman are suggesting. The results, it says, were disappointing.

A What Car? spokesman said: ‘Your dealer isn’t obliged to advise you on fuel economy beyond pointing at official MPG figures. However, considering the high extra costs involved, we had hoped that most dealers would be more candid.

‘We weren’t banking on them to quote the What Car? true MPG, but nor did we expect so many to be unrealistic. With a typical £308 a year at stake, buyers deserve better.’

The investigation comes as the price of unleaded is almost double where it stood at the turn of the millennium. It means working out potential fuel costs is more important than ever before for car owners.

Dacia Duster 1.6-litre 2WD

Price (starting from):£9,495Official MPG: 39.8True MPG: 26.1

What Car? says drivers will pay £931 a year more in fuel if they believe the official line. Most dealers it approached suggested it would get much nearer to 30mpg than the stated 39.8mpg. Overall, their estimates ranged from 32 to 35mpg.

Toyota Yaris 1.5 Hybrid

Price:£16,595Official MPG: 76.3True MPG: 47.7

Yaris owners could end up spending around £557 more per year on petrol if they follow the ‘official’ MPG figure. One dealer even told What Car? he knew someone who’d achieved 92mpg. Its first approaches to eight Toyota dealers prompted a raft of unhelpful ‘economy will depend on how and where you drive’ responses.

Nissan Note 1.5 dCi

Price:£14,100Official MPG: 78.5True MPG: 55.2

Only four of eight Nissan dealers What Car? contacted replied about the Nissan Note 1.5 dCi. Its headline-grabbing MPG figure claims an average of 78.5mpg. In its true test, it managed 55.2mpg. Of the four, one said to expect an MPG in the 70s. Two others reckoned between 55 and 60mpg, while another reported that his demonstrator car was delivering 60mpg.

Renault Captur 0.9TCe

Price:£14,495Official MPG: 56.5True MPG: 34.3

The three-cylinder petrol Captur claims to do an impressive 56.5mpg. In the true MPG tests it returns 34.3mpg. At current petrol prices that shortfall equates to £811 more than expected. Dealers that What Car? approached were more realistic about the car’s performance. One suggested deducting 10 per cent from the official figure while one said he commuted to work in the same model and was getting 41mpg.

Ford Focus 1.0T Ecoboost

Price:£16,245Official MPG: 58.9True MPG: 38.9

The shortfall between the official and true figures will cost drivers more than £600 in extra petrol. One dealer said he’d managed 51.2mpg in the showroom’s demo Focus and 48.6mpg on a different trip. Another said to expect economy in the mid to high 40s. What Car? was particularly heartened by the openness of a further sales person who reported she was getting about 38mpg in her Focus Ecoboost company car.

Volkswagen Golf 1.6 TDI Bluemotion

Price:£19,600Official MPG: 88.3True MPG: 56.8

The eye-catching 88.3mpg for this Golf equates to 56.8mpg in the What Car? test. Half of VW dealers it contacted gave no indication what MPG figure to expect, two offered test drives and a further two sent links to government websites explaining how official tests are conducted. A further two suggested knocking 5mpg off the claimed figure, while another was adamant that the official combined figure of 88.3mpg was achievable, even in town.

Audi A3 Sportback 2.0 TDI

Price:£24,995Official MPG: 68.9True MPG: 54.1

Audi sales people didn’t give What Car? much to go on. Half didn’t respond, while the rest suggested taking a test drive. One dealer stuck steadfastly to the official figures. It says if this car hadn’t been tested for its true MPG, motorists would have no reason to guess its true 54.1mpg would cost £300 more.

Peugeot 5008 1.6 HDI

Price: £19,045Official MPG: 56.4True MPG: 36.9

Half of dealers told What Car? the official 56.4mpg figure was realistic and achievable. However, the rest were more helpful. One sent details explaining how government tests are conducted and said that, as a rule, drivers should knock 20 per cent off the official figure. Other gave similar advice – with estimates ranging from the mid to the high 40s.

HOW IS THE MPG FIGURE WORKED OUT?

The biggest difference between the What
Car? MPG test and the official one is that the motor magazine uses
real-world conditions rather than in a laboratory.

What Car? editor Jim Holder said: 'The tests manufacturers submit their cars for is called the New European Driving Cycle.

'It is mandated at governmental level across Europe. It's important to be clear that the manufacturers didn't create the testing process - they just have to use it.

'The shortcoming is that it is conducted in a laboratory and in very strict/restricted circumstances which are virtually impossible to repeat.

'Vehicles are tested when cold, and usually on a rolling-road (roller test bench) or where there's no chance of the wind affecting the cycle. All electrical equipment and ancillaries like air-con are switched off.

'Our tests are on real roads in real conditions - hence they are far more accurate for the vast majority of motorists.'